Prior art systems for manufacturing printed circuit boards employ either a "subtractive" or an "additive" process. The starting material in the subtractive process is an insulating substrate which has a relatively thick layer of copper laminated to one or both of its sides. An etch-resistant material or "resist," typically in ink form, is applied by silk-screening or a similar technique over each copper layer in a positive image of the desired circuit configuration to be fabricated thereon.
Alternatively, to yield greater circuit density through increased resolution (i.e., narrower trace width) and uniform reduced spacing between adjacent traces, photographic techniques are used in which a photo-sensitive resist (so-called "photo-resist") is uniformly disposed over each copper layer. An appropriate photographic transparency is then made in the form of a negative of the desired circuit configuration for each side of the board. Each side is exposed through its corresponding transparency to intense ultra-violet light. Those regions of the photo-resist which were exposed to this light polymerize and become etch-resistant, so that a positive latent image of the desired circuitry is formed in the photo-resist after exposure.
Once the desired circuit pattern is formed in the resist, regardless of the specific technique used, the board is immersed in a copper etchant bath which removes all the copper not protected by the resist or polymerized photo-resist. When the etching is complete, the board is removed from the bath and the remaining resist is removed, leaving the laminated copper on each side in the form of the desired circuit configuration.
There are a number of disadvantages with the subtractive printing processes. First, the boards produced by this process have significant material costs because they use expensive photochemicals, resists and etchants and waste substantial amounts of copper. Second, the trace width is limited by "undercutting," i.e., lateral undermining of the traces under the resist. Third, this process is highly polluting because the unwanted copper must be chemically etched from the substrate. Fourth, this process is time consuming, typically requiring 90 minutes or more to manufacture a printed circuit board. Fifth, capital equipment and labor costs associated with subtractive processes are substantial.
There are two general types of additive processes, "thin film" and "thick film," both of which start with an unclad insulating substrate. In the thin film process an activating agent, which promotes adhesion of copper to the substrate, is applied over the entire surface thereof. A mask is then applied to the substrate and the substrate is immersed in an electroless copper plating bath which deposits copper onto all the non-masked areas including the walls of the pre-drilled holes. Once plating is completed, the board is removed from the bath and a protective coating (e.g., a solder mask) is applied over each plated side of the board.
There are also a number of disadvantages with prior art thin film additive processes. First, substantial time is required to electrolessly deposit a copper coating of sufficient thickness on the substrate to form conductors. Second, once this coating is deposited it adheres extremely poorly to the substrate, i.e., the plated copper traces easily peel off the substrate during subsequent manufacture or use of the board. Third, electrolessly deposited copper has inferior physical properties compared with electrolytically deposited copper. For example, the conductivity of electrolessly deposited conductors is substantially less than the conductivity of electrolytically deposited conductors.
In the thick film process conductive particles are suspended in a binder or "carrier," forming a viscous "ink." The ink is then screened onto the substrate in the desired pattern and dried, causing the carrier to vaporize. The remaining metal particles are then "fired" or sintered at high temperature into conductors.
One such thick film process is disclosed in U.S. Pat. No. 4,278,702. In this process a paste comprising metallic particles suspended in a viscous carrier is applied to a substrate using a conventional silk screen process in which openings have been formed in the screen in the desired circuit pattern. Thereafter, a high frequency electromagnetic field is applied to the substrate which heats the metal particles to a sufficient temperature to sinter them. This process has a number of disadvantages. First, since the sintered particles are joined only in the proximity of their surfaces, the conductors are not mechanically strong and may be ruptured easily when subjected to modest mechanical stress. Second, the sintered conductors have lower conductivity than solid metal conductors. Third, sintered conductors are more subject to chemical corrosion than solid conductors.
Another prior art additive process is disclosed in U.S. Pat. No. 5,014,420. In this process metal particles of substantially uniform surface area are applied to a substrate either as a powder or ink using electrostatic forces. The particles are then fused into solid conductors using a highly concentrated, high frequency electromagnetic field which sequentially scans across the surface of the substrate. This process also suffers from a number of deficiencies. First, since the force on the particles varies as the square of the flux density and this force tends to drive the particles out of the concentrated electromagnetic field, it is difficult to control the location of the resulting conductors on the substrate. Second, since the electromagnetic field is scanned across the substrate, excessive time is needed to cure the conductors. Third, since particles of substantially uniform size are used, there is considerable shrinkage in the deposited traces when the particles are melted, which could lead to unwanted pattern distortion or breaking up of the conductive traces.
Accordingly, one object of the present invention is to provide a new and improved apparatus for and method of manufacturing printed circuit boards.
Another object of the invention is to provide a new and improved system for additive printing of printed circuit boards.
Still another object of the present invention is to provide a new and improved apparatus for and method of rapidly making printed circuit boards.
Yet another object of the present invention is to provide a new and improved apparatus for and method of making printed circuit boards with minimal polluting byproducts.
Still another object of the present invention is to provide an apparatus and method for curing metallic particles deposited on a dielectric substrate into solid electrical conductors thereon.
Still another object of the present invention is to provide an apparatus and method for making printed circuit boards inexpensively.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.